An image sensor is a semiconductor device for converting an optical image into an electrical signal. Image sensors may be chiefly classified into charge-coupled devices (CCDs) and CMOS Image Sensors (CIS).
A CCD image sensor, however, can be disadvantageous in terms of driving complexity, high power consumption and complexity in fabrication process by requiring numerous masks processing steps.
The CMOS image sensor, on the other hand, has qualities in which the defects of the CCD may not be present. A CMOS image sensor may include a photodiode and a MOS transistor within a unit pixel. The CMOS image sensor sequentially detects an electrical signal of each unit pixel in a switching way to realize an image.
A vertical CMOS image sensor is a type of CMOS image sensor that can be fabricated without using color filter layers. The lack of color filter layers by the vertical CMOS image sensor is accomplished by varying the thickness of a light shielding layer different using a wavelength-dependent absorption degree and varying the transmission depth of light.
Vertical CMOS image sensors offer an advantage such as the lack of a color filter layers or a microlens and realizes all of red (R), green (G), and blue (B) colors through the use of a single pixel.
As illustrated in example FIG. 1A, in a vertical CMOS image sensor, ions may be implanted in first epitaxial layer 10 to form red photodiode 12 in first epitaxial layer 10. Second epitaxial layer 20 may then be formed on and/or over first epitaxial layer 10. Ions may then be implanted to form first plug 22 in second epitaxial layer 20. Ions may then be simultaneously implanted in first contact region 24 and also to form green photodiode 26 on and/or over the second epitaxial layer 20. Third epitaxial layer 30 may be formed on and/or over second epitaxial layer 20 and ions may be implanted to form second plug 32 in third epitaxial layer 30.
Device isolation film 38 is formed in third epitaxial layer 30. Ions may then be implanted to form blue photodiode 34 in third epitaxial layer 30, and second contact region 36 can be formed simultaneously formed.
Such vertical CMOS image sensors have several disadvantageous. Firstly, they may experience contact defect (B) at first plug 22. Consequently, contact defect (B) does not allow the flow of an electron-hole pair formed by light collection, thereby causing a noise or inducing a phenomenon of shortage of current.
Secondly, the vertical CMOS image sensor has a curved profile of first plug 22 which results in a curve (A) at a depletion region. Accordingly, green photodiode 26 and first plug 22 should be formed with a large gap therebetween, thereby increasing the size of the CMOS image sensor.
Thirdly, as illustrated in example FIG. 1B, the vertical CMOS image sensor experiences crystal defect (C) in an epitaxial layer growth process due to the epitaxial layer growth process being performed twice.
Finally, the vertical CMOS image sensor requires several additional process steps such as the epitaxial layer growth process, the plug pattern process, the plug ion implantation process.